Semiconductor crystal growth method

ABSTRACT

A substrate is heated in a crystal growth vessel evacuated to a ultrahigh vacuum, and gases containing component elements of a crystal to be grown on the substrate are introduced into the vessel under predetermined conditions to cause successive epitaxial growth of single molecular layers, the number of growth cycles being automatically controlled. A mass analyzer is disposed opposite to the substrate in the vessel so that the progress of crystal growth can be incessantly traced and evaluated for each of the molecular layers. An etchant gas introduction nozzle extends into the vessel to make etching treatment of the surface of the substrate in the evacuated vessel prior to the crystal growth.

This is a division of application Ser. No. 08/022,690, filed Mar. 1,1993, pending, which is a continuation application of Ser. No.07/794,344, filed Nov. 12, 1991, now abandoned, which is in turn acontinuation application of Ser. No. 07/234,001, filed Aug. 12, 1988,now abandoned, and which is in turn a continuation application of Ser.No. 06/759,111, filed Jul. 25, 1985, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the invention

The invention relates to a semiconductor crystal growth apparatus suitedfor forming monocrystalline growth layers of a semiconductor withprecision as precise as a single molecular layer.

2. Description of the Prior Art

A Metal Organic Vapour Phase epitaxy process (hereinafter referred to asan MO-CVD process), a molecular beam epitaxial process (hereinafterreferred to as an MBE process) and an atomic layer epitaxial process(hereinafter referred to as an ALE process) are well known in the art asvapor phase epitaxial techniques for obtaining crystalline thin film ofsemiconductors.

In the MO-CVD process, III and V group elements as sources, and hydrogengas or the like as a carrier are simultaneously introduced into areaction chamber to cause crystal growth by means of thermaldecomposition. The thermal decomposition results in a poor quality ofthe grown crystal layer. In addition, the thickness control which isdimensionally as precise as a single monolayer is difficult.

The MBE process is well known as a crystal growth process making use ofa ultrahigh vacuum, This process, however, includes a first stage ofphysical adsorption. Therefore, the quality of the crystal obtained isinferior to that obtained by the CVD process which makes use of achemical reaction. Besides, for the growth of a compound semiconductorsuch as GaAs of III and V group elements, III and V group elements areused as sources and are disposed in a growth chamber. Therefore, it isdifficult to control the amount and rate of vaporization of gasesevaporated as a result of the heating of the sources. In addition,replenishment of the sources is difficult. Further, it is difficult tomaintain a constant growth rate for a long period of time. Furthermore,the evacuating device is complicated in construction. Still further,precise control of the stoichiometric composition of a compoundsemiconductor is difficult. Consequently, the MBE process is defectivein that high quality crystals cannot be obtained.

The ALE process is an improvement over the MBE process. In this process,component elements of a compound semiconductor are alternately suppliedin the form of pulses so that monoatomic layers are alternatelydeposited on a substrate to cause growth of a thin film composed ofatomic layers, as disclosed in U.S. Pat. No. 4,058,430 (1977) to T.Suntola et al. Although this process is advantageous in that the filmthickness can be controlled with the precision of the atomic layer, itis actually an extension of the MBE process, and the crystal quality isnot satisfactory as in the case of the MBE process. Besides, itsapplication is limited to growth of thin films of compoundsemiconductors, e.g., those of II and IV group elements, such as CdTeand ZnTe, and the process is not successfully applicable to Si or GaAs,which is the most important semiconductor material presently used forthe production of semiconductor devices including ultra LSI's. There areattempts for improving the ALE process so as to absorb molecules to thesurface of a crystal thereby to make use of chemical reactions on thesurface of the crystal. This approach, however, concerns only with thegrowth of polycrystals of ZnS or amorphous thin films of Ta₂ O₅, and hasnot concern with a single crystal growth technique.

With any of the prior art crystal growth processes described above, ithas been difficult to obtain a crystal film of high quality and it hasnot been easy to control the thickness of the crystal film to a desiredvalue.

In the meantime, in the manufacture of a semiconductor device, it isimportant to make evaluation during the manufacturing process as towhether or not a crystal is growing as designed for obtaining ahigh-quality semiconductor device. In the prior art, the evaluation hasbeen done by taking out the semiconductor from the growth vessel andtesting it using an analyzer. The operation of evaluation, therefore,has been very cumbersome, and the evaluation efficiency has not beenhigh, resulting in incapability of attaining satisfactory qualitycontrol. Further, when a new device is to be manufactured, theevaluation has required a long time resulting in a great delay of themanufacture.

From the aspect of growth of a thin crystal film on a substrate, thesurface state thereof is very important. If the surface state isunsatisfactory, the grown crystal will also have an unsatisfactorycrystal property, and, in worst cases, no crystal growth is attained atall. In the case of a GaAs substrate, for instance, it has to bepretreated, prior to the crystal growth, by means of wet etching using aliquid etchant mixture consisting of H₂ SO₄, H₂ O₂ and H₂ O. However,since the surface after etching is very active, an oxide layer or likedeposit layer has been formed thereon, if it were exposed to atmosphereafter the step of etching.

SUMMARY OF THE INVENTION

An object of the invention is to provide a semiconductor crystal growthapparatus, which can obviate the prior art drawbacks noted above and canautomatically form high-quality single crystal layers with precision asprecise as a single molecular layer.

According to one aspect of the invention which attains the above object,there is provided a semiconductor crystal growth apparatus, whichcomprises a crystal growth vessel for accommodating a substrate, heatingmeans for heating the accommodated substrate, evacuating means forevacuating the crystal growth vessel to a ultrahigh vacuum, nozzle meansfor introducing gases containing component elements of a crystal to begrown on the substrate into the crystal growth vessel from outside,valve means provided between the nozzle means and sources of the gases,and control means for controlling the opening and closing of the valvemeans according to a preset open-close time chart and a preset number ofcycles of valve opening and closing.

The apparatus having such a construction can ready cause successivegrowth of molecular layers which satisfy the desired stoichiometricalcomposition, so that a high-quality crystal can be obtained. Inaddition, since impurities can be doped in the desired layers, it ispossible to obtain a very sharp impurity concentration distribution.Further, an epitaxial growth layer having a desired thickness can beobtained automatically with precision as precise as a single molecularlayer.

Another object of the invention is to provide a semiconductor crystalgrowth apparatus, which can manufacture a semiconductor with highefficiency by successively tracing and evaluating the progress ofsemiconductor crystal growth.

According to another aspect of the invention which attains this object,there is provided a semiconductor crystal growth apparatus, whichcomprises a crystal growth vessel for accommodating a substrate, heatingmeans for heating the accommodated substrate, evacuating means forevacuating the crystal growth vessel to a ultrahigh vacuum, nozzle meansfor introducing gases containing component elements of a crystal to begrown on the substrate into the crystal growth vessel from outside, anda mass analyzer disposed opposite to the accommodated substrate.

With this apparatus having the mass analyzer provided in the crystalgrowth vessel, the progress of semiconductor crystal growth can beinstantaneously evaluated, so that it is possible to manufacture asemiconductor device with high efficiency.

A further object of the invention is to provide a semiconductor crystalgrowth apparatus, which can cause growth of a high-qualitymonocrystalline film on a substrate with dimensional precision asprecise as a single molecular layer, by etching the substrate surface ina vacuum prior to the crystal growth.

In accordance with another aspect of the invention which attains thisobject, there is provided a semiconductor crystal growth apparatus,which comprises a crystal growth vessel for accommodating a substrate,heating means for heating the accommodated substrate, evacuating meansfor evacuating the crystal growth vessel to a ultrahigh vacuum, nozzlemeans for introducing gases containing component elements of a crystalto be grown on the substrate, and another nozzle means for introducingan etchant gas.

With this apparatus, the etching process which is a pretreatment priorto the epitaxial growth can also be executed in the same crystal growthvessel. The substrate surface can thus be pretreated to a statesatisfactory for the crystal growth. That is, growth of a satisfactorysingle crystal which satisfies the desired stoichiometrical compositioncan be reliably attained. It is thus possible to obtain a semiconductordevice having very satisfactory characteristics.

Other objects and features of the invention will become more apparentfrom the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing an embodiment of the semiconductorcrystal growth apparatus according to the invention.

FIG. 2 is a graph showing a relation between the thickness of the growthfilm and the number of times of valve opening and closing in theapparatus shown in FIG. 1.

FIG. 3 is a flow chart illustrating the operation of the control unitshown in FIG. 1.

FIG. 4 is a time chart showing the timing of gas introduction in theapparatus shown in FIG. 1.

FIG. 5 is a schematic view showing another embodiment of thesemiconductor crystal growth apparatus according to the invention.

FIG. 6 is a waveform diagram showing levels of compounds detected by themass analyzer shown in FIG. 5.

FIG. 7 is a schematic view showing a further embodiment of thesemiconductor crystal growth apparatus according to the invention.

FIG. 8 is a schematic view showing a still further embodiment of thesemiconductor crystal growth apparatus according to the invention.

FIG. 9A is a graph showing the relation between the rate of etching ofthe GaAs substrate with GaCl₃ and the substrate temperature.

FIG. 9B is a graph showing the relation between the etching rate and theGaCl₃ supply rate.

FIG. 10 is a schematic view showing yet another embodiment of thesemiconductor crystal growth apparatus according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, a crystal growth vessel 1 is made of stainlesssteel or like metal. The vessel 1 is coupled to an evacuating unit 3 viaa gate valve 2 for evacuating its interior to a ultrahigh vacuum. Thevessel 1 includes nozzles 4 and 5 for introducing gaseous compoundscontaining III and V group elements respectively as components of aIII-V group compound semiconductor which should grow on a substrate 12.The nozzles 4 and 5 are provided with on-off valves 6 and 7 forcontrolling the introduced amounts of the gaseous compounds 8 and 9containing the III and V group elements, respectively. A heater 10 forheating the substrate 12 is disposed in the vessel 1, and a thermocouple11 is coupled to the heater 10 for measuring the temperature thereof ofthe substrate 12. The heater 10 includes a tungsten filament sealed in aquartz glass casing on which the substrate 12 of a compoundsemiconductor is mounted. The vessel 1 is further provided with apressure gauge 13 for measuring the value of its internal vacuum. Thevessel 1 is further provided with an optical window 15, through whichradiation having a specific wavelength emitted from a radiation source14 is focused on the substrate 12. Electromagnetic valves 16 and 17 areprovided for controlling the flow of compressed air which is used toopen and close the valves 6 and 7. The electromagnetic valves 16 and 17are on-off controlled by a control unit 18 utilizing a microcomputer. Adisplay unit 19 is provided to display the number of cycles of openingand closing the valves 6 and 7.

A monocrystalline thin film of a compound semiconductor is formed in amanner as described below by the crystal growth apparatus of the abovestructure. Suppose, for example, the case of epitaxial growth of asingle crystal of GaAs on the GaAs substrate 12. First, the vessel 1 isevacuated to about 10⁻⁷ to 10⁻⁸ Pascal (hereinafter abbreviated as Pa)by opening the gate valve 2 and operating the ultrahigh-vacuumevacuating unit 3. Then, the GaAs substrate 12 is heated to 300° to 800°C. by the heater 10. Thereafter, gaseous trimethyl gallium (TMG) 8 isintroduced as Ga-containing gas by holding the valve 6 open for 0.5 to10 sec. and maintaining the internal pressure of the vessel 1 at 10⁻¹ to10⁻⁷ Pa. Then, the valve 6 is closed, and the vessel 1 is evacuatedagain. Thereafter, gaseous arsine (ASH₃) 9 is introduced as a gascontaining As by holding the valve 7 open for 2 to 200 sec. andmaintaining the internal pressure of the vessel at 10⁻¹ to 10⁻⁷ Pa. As aresult, at least one molecular layer of GaAs grows on the substrate 12.

In this case, when the substrate 12 is irradiated with ultraviolet raysemitted from the radiation source 14 while it is heated, the growthtemperature can be reduced to be 400° C. or below, and the crystalquality can be improved.

It is possible to successively form molecular layers of GaAs having thesame thickness by executing the epitaxial growth by setting the internalpressure of the growth vessel 1, the temperature of the substrate 12,the intensity of radiation from the radiation source 4 and the on-offdurations of the valves 6 and 7 at predetermined values, respectively.Thus, growth of an epitaxial growth layer of GaAs having a desiredthickness can be attained with precision as precise as a singlemolecular layer by repeating the molecular-layer growth cycle apredetermined number of times.

FIG. 2 shows the experimentally observed relation between the thicknessof the GaAs epitaxial layer and the number of cycles of alternatelyintroducing gaseous TMG and AsH₃ at a growth temperature of 500° C. Forexample, epitaxial layers having film thicknesses of 1,100 Å, 0.57 μmand 1.13 μm grew as a result of 400, 2000 and 4000 cycles of alternateintroduction of the gases 8 and 9, respectively. It should be noted thatthe relation between the thickness of the growth film and the number ofcycles of gas introduction, i.e., the number of cycles of opening andclosing the valves, is very linear. It is thus confirmed that the filmthickness of the growth layer can be controlled by controlling thenumber of cycles of opening and closing the valves.

The electromagnetic valves 16 and 17 and control unit 18 are providedfor controlling the film thickness of the growth layer by controllingthe number of cycles of opening and closing the valves. Data indicativeof the number of cycles of opening and closing the valves 6 and 7, theperiod τ₁ of introduction of the gas 8, the period τ₂ of exhausting thegas 8, the period τ₃ of introduction of the gas 9 and the period τ₄ ofexhausting the gas 9, are preset in the control unit 18 incorrespondence to the thickness of the growth layer to be obtained.

The control unit 18 includes a clock pulse generator, various countersand registers. When the operation of the control unit 18 is started, itcontrols the opening and closing of the valves 6 and 7 with timing asshown in FIG. 4 by running a routine as shown in FIG. 3.

Referring to FIG. 3, the control unit 8 first initializes the variousinternal counters and registers (step 100). Then, it applies an "on"signal to the electromagnetic valve 16 to turn on (i.e., open) the valve6 at time t₁. The control unit 18 resets a time counter (τ) and thenre-starts the time counting operation of the time counter (step 101).The gas 8 is thus introduced for crystal growth on the substrate 12.When the control unit 18 detects that the count τ of the time counter,i.e., the gas introduction period, attains τ₁ (step 102), it turns offthe valve 6 at time t₂, thereby exhausting the introduced gas 8. Thecontrol unit 18 resets the time counter (τ) once and re-starts the timecounting operation of the time counter (step 103). After the presetexhausting period τ₂ has elapsed (step 104), the control unit 18 turnson the valve 7 at time t₃, and re-starts the time counter in the manneras described (step 105). At this time the gas 9 is now introduced forcrystal growth on the substrate 12. When the gas introduction period τattains τ₃ (step 106), the control unit 18 turns off the valve 7 at timet₄, thereby exhausting the gas from the vessel 1 and re-starts the timecounter (step 107). After the preset exhausting period τ₄ has elapsed(step 108), the count N of a valve on-off cycle counter is incrementedto N+1. By the above sequence of operations of the control unit 18, onemolecular layer of GaAs is formed on the substrate 12 as mentionedearlier. The control unit 18 subsequently checks as to whether or notthe count N has attained the preset number No (step 110). If the presetnumber No has not yet been reached, the control unit 18 repeats thesequence of operations to form a second molecular layer of GaAs. Whenthe predetermined successive molecular layers of GaAs are formed on thesubstrate 12, the growth operation is ended. In this way, the film ofGaAs having the desired thickness grows automatically on the substrate12 with the precision as precise as a single molecular layer. While thecontrol unit 18 is performing the sequence of control operations notedabove, the number N of executed valve on-off cycles is displayed on thedisplay unit 19. The operator participating in the manufacture of thesemiconductor thus can grasp the progress of the crystal growth.

The Ga-containing material gas may be gaseous TMG, ZEGaCl, GaBr₃, GaI₃or GaCl₃. The irradiation with the ultraviolet radiation may be donecontinuously or intermittently during the process of growth. Theradiation source may be a lamp such as a high-pressure mercury lamp or axenon lamp or a laser as an excimer laser.

While the above embodiment has referred to GaAs as a semiconductor to begrown as a crystal, the invention is of course applicable to other III-Vgroup compounds and II-VI group compounds such as InP, AlP, GaP, etc.Further, it is possible to grow mixture crystals such as Ga.sub.(1-x)Al_(x) As and Ga.sub.(1-x) Al_(x) As.sub.(1-y) P_(y). Further, thesubstrate is not limited to GaAs, but it is possible to causeheteroepitaxial growth on substrates of other compounds. Further, thesemiconductor may be an element semiconductor belong to, for example,the IV group. Where the element semiconductor is Si, the crystal growthmay be caused by using a combination of such a chloride as SiCl₄, SiHCl₃and SiH₂ Cl₂ and H₂ gas.

Further, while, in the above embodiment, the heat source for heating thesubstrate 12 has been provided inside the growth vessel 1, it is alsopossible to dispose an infrared lamp or like heat source outside thegrowth vessel 1 so that the substrate 12 may be heated by heat raystransmitted through an optical window provided on the vessel 1 toirradiate the substrate 12.

FIG. 5 shows another embodiment of the semiconductor crystal growthapparatus according to the invention. In the FIG. 5, the same referencenumerals are used to designate the same or equivalent parts appearing inFIG. 1. This apparatus is greatly different from the structure shown inFIG. 1 in that a mass analyzer 24, a mass analyzer controller 25, amultiple ion sensor 26 capable of simultaneously sensing a plurality ofdifferent kinds of molecules, and a multiple pen recorder 27 forrecording the output of the multiple ion sensor 26, these componentsconstituting evaluating means. The other structure of the apparatusexcept that concerning the evaluating means is the same as that in thepreceding embodiment shown in FIG. 1, so it will not be described anyfurther.

With this apparatus, epitaxial growth of molecular layers is attainedwhile tracing and evaluating the progress of crystal growth by theevaluating means 24 through 27 as follows. Suppose, for example, thatGaAs is a semiconductor whose crystal grows on the substrate 12; gaseoustrimethyl gallium (TMG), which is a III group compound, is the gas 8 tobe introduced; and gaseous arsine (ASH₃), which is a V group compound,is the gas 9.

First, the substrate 12 is set in the growth vessel 1, and the vessel 1is evacuated by the evacuating unit 3 to about 10⁻⁷ to 10⁻⁸ Pascal(hereinafter referred to as Pa). Then, the operation of the evaluatingmeans 24 through 27 is started. The peak selector of the multiple ionsensor 26 is set to select the introduced gas molecules of AsH₃(M/e=78), and TMG (M/e=114) and to select the reaction product moleculesof, for example, CH₄ (M/e=16). Then, the GaAs substrate 12 is heated to300° to 800° C., for instance, by the heater 10. Gaseous TMG 8 is thenintroduced as Ga-containing gas by holding the valve 6 open for 0.5 to10 seconds and maintaining the internal pressure of the growth vessel 1at 10⁻¹ to 10⁻⁷ Pa. Subsequently, the valve 6 is closed, and the gas inthe vessel 1 is exhausted. Now, gaseous AsH₃ 9 is introduced asAs-containing gas by holding the valve 7 open for 2 to 200 seconds andmaintaining the internal pressure of the vessel 1 at 10⁻¹ to 10⁻⁷ Pa. Inthis way, at least one molecular layer of GaAs grows on the substrate12. It is to be noted that a single crystal growth layer of GaAs havinga desired thickness grows with precision as precise as a singlemolecular layer by repeating the sequence of operations described above.

In the above crystal growth process, by alternately introducing gaseousTMG and ASH₃, not only TMG and AsH₃ but also methane (CH₄) which is areaction product can be detected, and the progress of the crystal growthcan be successively traced by the multiple ion sensor 26.

FIG. 6 shows the relative intensities of AsH₃ (M/e=78), TMG (M/e=114)and CH₄ (M/e=16) detected by the multiple ion sensor 26 when TMG andAsH₃ are alternately introduced in the manner as described above. Morespecifically, the graph shows the data obtained when the step ofintroducing TMG under a pressure of 10⁻¹³ Pa by holding the valve 6 openfor 2 seconds and the step of introducing AsH₃ under a pressure of 10⁻²Pa by holding the valve 7 open for 10 seconds are alternately repeated.

Where TMG or GaCl₃ is used as the Ga-containing compound, the introducedgas will attach to the wall of the growth vessel as well as to thesubstrate because of great interaction of the gas with the vessel walldue to its low vapor pressure at room temperature. Further, theattaching compound is liberated with the lapse of time, and theliberated gas cannot be distinguished from the gas liberated from thesubstrate.

FIG. 7 shows a further embodiment or a modification, which can solve theabove problem. In this apparatus, a shroud 28 is provided in such aposition as to surround the nozzles 4 and 5. The shroud 28 has holes oropenings aligned with the respective nozzles. To cool the shroud 28, acoolant reservoir 29 is provided at an end of the shroud, and a coolant31 is poured into the reservoir 29 through an inlet 30 thereof.Likewise, another shroud 32 is provided, which has a detection openingwhich is aligned with the end of mass analyzer 24, and another coolantreservoir 33 is provided at an end of the shroud 32 to cool the samewith a coolant 31 poured into the reservoir 33 through an inlet 34.Means for cooling the shrouds is not limited to a coolant such asliquefied nitrogen, and a miniature freezer can also be utilized.

With this arrangement, an excess of the gas introduced into the growthvessel 1 is adsorbed by the shrouds 28 and 32, and only molecules thatare liberated from the substrate 12 are captured by the mass analyzer24. Thus, it is possible to attain accurate analysis of the growthlayer.

A high-quality semiconductor device can be manufactured efficiently byincessantly tracing and evaluating the progress of growth of onemolecular layer after another of a semiconductor crystal on thesubstrate 12, by the mass analyzer mounted to the growth vessel 1.

It is needless to mention that the material of the substrate 12 and thesemiconductor formed thereon is not limited to GaAs. In addition, it ispossible to introduce more than two different kinds of gases into thegrowth vessel 1, by increasing the number of nozzles for doping withimpurities or obtaining mixed crystals.

In the above embodiments, the heat source for heating the substrate 12is disposed in the growth vessel 1, but it is also possible to providean infrared lamp or the like disposed outside the growth vessel 1.Further, the substrate 12 may be irradiated with light while it isheated. By so doing, it is possible to reduce the substrate temperatureand further improve the quality.

FIG. 8 shows a further embodiment of the semiconductor crystal growthapparatus according to the invention. This apparatus is provided with amechanism which permits vapor phase etching as means for treating thesubstrate surface. In FIG. 8, the same reference numerals are used todesignate the same or equivalent parts appearing in FIG. 1. Thisapparatus is different from the apparatus shown in FIG. 1 in that itdoes not have the radiation source 14 and optical window 15 forirradiate the substrate 12 with radiation from outside the growth vessel1 and, in lieu thereof, it is provided with a nozzle 40 for introducinga gaseous compound 42 used for the vapor phase etching and a valve 41for opening and closing the nozzle 40 introducing the gaseous compound42 used for the vapor phase etching. The other parts except thoseprovided for introducing the etching gas are the same as those describedbefore in connection with FIG. 1, so they will not be described anyfurther.

In this apparatus, the vapor phase etching is carried out as follows.Suppose, for example, the case, in which a GaAs substrate is used as thecompound semiconductor substrate and GaCl₃ is used as the gaseouscompound introduced for etching purpose. First, the GaAs substrate 12 isetched in the usual way, then rinsed and dried, and then set on theheater 10. Subsequently, the growth vessel 1 is exhausted toapproximately 10⁻⁷ Pa by the exhausting unit 3. Then, gaseous GaCl₃ isintroduced to provide an internal pressure of about 10⁻⁶ to 10⁻⁵ Pa byopening the valve 41. The GaAs substrate could be etched at a rate ofabout 1 to 1,000 Å/min by varying the substrate temperature.

FIG. 9A shows the relation between the etching rate and the substratetemperature when the quantity of supply of GaCl₃ is taken as aparameter. The curve A is obtained when the quantity of supply of GaCl₃is one-third the quantity supplied in the case of the curve B. FIG. 9Bshows the relation between the etching rate and the GaCl₃ supplyquantity when the substrate temperature is taken as a parameter. Thecurve C is obtained when the substrate temperature is 350° C., while thecurve D is obtained when the substrate temperature is 250° C.

It will be seen from these graphs that the etching rate is determined bythe quantity of GaCl₃ supply while the substrate temperature is high,but when the substrate temperature is reduced, the etching rate issubstantially independent of the quantity of GaCl₃ supply and dependsonly on the substrate temperature. Thus, an optimum etching rate can beset by suitably selecting the substrate temperature and the quantity ofGaCl₃ supply.

FIG. 10 shows a further embodiment of the invention. This apparatuswhich is a modification of that shown in FIG. 8 is provided with amechanism for irradiating the substrate 12, i.e., a radiation source 14and an optical window 15 for directing radiation when vapor phaseetching is carried out.

By the irradiation, it was possible to reduce the etching temperature by100 degrees or more. The irradiation may be made continuously orintermittently during the process of vapor phase etching. In this case,not only the substrate 12 is irradiated, but also the etching gas itselfmay be radiated by forming the nozzle from a transparent material. By sodoing, the etching gas can be activated to promote the etching process.The radiation source is not limited to a lamp such as a high-pressuremercury lamp or a xenon lamp; i.e., it is possible to employ an excimerlaser or a multiplied laser beam.

The activation of the etching gas may be caused not only by theirradiation noted above but also by applying a voltage to a highfrequency coil or electrodes provided in the neighborhood of the nozzle41.

While the above embodiments have referred principally to the (100) planeof GaAs used as the substrate for the crystal growth, the invention isapplicable to other planes as well and is not restricted in any way bythe impurity concentration of the substrate. Further, the invention isequally effectively applicable to II-VI group compounds, such as InP,AlP, GaP, etc., and also to IV group element semiconductors, such as,Si, Ge, etc. Further, the invention is applicable to mixed crystals,e.g., Ga.sub.(1-x) Al_(x) As, Ga.sub.(1-x) Al_(x) As.sub.(1-y) P_(y),etc. Further, the gaseous compound used for the etching is not limitedto GaCl₃, and it may be HCl, HBr, PCl₃, AsCl₃, Cl₂, SF₆, CCl₂ F₂, CF₄,C₃ F₈, CH₃ Br, etc. Further, it is apparent that a substratepretreatment chamber may be disposed adjacent to the crystal growthchamber, and the substrate may be pretreated in the pretreatment chamberbefore it is moved under vacuum into the crystal growth chamber forcrystal growth therein.

What is claimed is:
 1. A method of growing a semiconductor crystal on asubstrate through epitaxial growth of molecular layers, comprising thesteps of:heating a substrate within a crystal growth vessel enclosingsaid substrate; introducing gases containing component elements of acrystal to be grown on said substrate in said vessel from outsidesources via nozzles; permitting and stopping the introducing of thegases on said substrate by operating valve means in open and closedpositions, said valve means being arranged between said nozzles and saidsources; selectively cycling the opening and the closing of said valvemeans in accordance with a program having data indicative of a number ofcycles of opening and closing of the valve means, a time period forintroduction of each of the gases, and a time period for exhaustion ofeach of the gases between introductions of each of the gases, said databeing selected based on attaining a desired thickness of the growthlayer to be obtained as precise as a single molecular layer; exhaustingresiduals of the gases by evacuating an interior of said vessel with anultrahigh vacuum of 10⁻⁷ to 10⁻⁸ Pa even while said control means isselectively cycling the opening and closing of the valve means; tracingand evaluating progress of the semiconductor crystal growth within thevessel, said step of tracing and evaluating including extending a massanalyzer into said vessel for capturing molecules liberated from saidsubstrate; absorbing an excess of the gas being introduced, the step ofabsorbing including employing a first shroud within said vessel, saidfirst shroud having opening aligned with respective openings of saidnozzles, employing a second shroud having a detection opening which isaligned with an end of said mass analyzer, arranging said first andsecond shrouds for absorbing the excess of the gas to allow said massanalyzer to capture only molecules liberated from said substrate; andcooling both of said first and second shrouds.
 2. A method as in claim1, further comprising:irradiating said substrate with ultraviolet rayswhile said substrate is being heated, wherein the step of introducinggases takes place under pressure of 10⁻¹ to 10⁻⁷ Pa.
 3. A method as inclaim 2, wherein the step of irradiating includes irradiating with aradiation source means that is external to said vessel, said vesselhaving a window between said radiation source means and where saidsubstrate is within said vessel.
 4. A method as in claim 1, wherein saidsubstrate is arranged at an elevation within the vessel between that ofsaid nozzles and that where the exhausting takes place.
 5. A method asin claim 1, further comprising introducing an etchant gas within saidvessel to treat a surface of said substrate with vapor phase etchingfrom said etchant gas before growing the crystal.
 6. A method as inclaim 5, further comprising arranging a high frequency coil orelectrodes in the neighborhood of at least part of said etchant gasintroduction nozzle means.
 7. A method as in claim 5, further comprisingirradiating at least part of said etchant gas.